The present invention relates to a system for driving a semiconductor laser, whereby the power of laser light emitted from the semiconductor laser or a light source can be suitably controlled depending upon the readout or reproduction mode or the write or recording mode.
There has been invented and demonstrated an optical recording and reproducing device or system in which the light beam from a light source and more particularly the laser light emitted is focused into a light spot of less than one micrometer in diameter on a photosensitive medium vacuum deposited or otherwise formed on a surface of a disk-like base so that the video signal or the digital signal can be recorded in or reproduced from concentric circular or spiral tracks arranged in accordance with a predetermined pattern or in a random order on the disk. With the optical recording and reproducing device or system of the type described, it has been well known in the art that the power or intensity of laser light varies over a wide range depending upon the environmental temperature so that it has been extremely difficult to stabilize the intensity of laser light constantly regardless of the environmental temperature. To put into another way, there has been a strong demand of long standing for a semiconductor system which can emit the laser light with a predetermined intensity regardless of the environmental temperature. In order to solve this problem, there has been invented and used a system in which a feedback system is provided in such a way that in response to the output from a photosensor or the like which is adapted to sense the intensity of light emitted from a semiconductor laser in which light is reflected back and forth in an optical resonator, the current for driving the laser is controlled, whereby the intensity of the laser light emitted can be maintained at a desired level.
However, if the laser light power or intensity control system of the type described above is employed in an optical recording and reproducing device in which a semiconductor laser is used as a light source and the power or intensity of light emitted from the semiconductor laser is switched depending upon the readout or reproduction mode or the write or recording mode, there arise a variety of problems or drawbacks. In the case of the optical recording and reproducing device with a semiconductor laser, the power P.sub.R of the laser light emitted from the semiconductor for readout or reproduction is considerably different in level from the power P.sub.W of laser light used in writing or recording data or information. For instance, in the case of an optical recording and reproducing disk rapidly spinning at a speed of, for instance, 1800 rpm, the write or recording power P.sub.W must be increased by about 5 to 10 times as high as the readout power P.sub.R. If the power control circuits or the like are employed for controlling the readout power P.sub.R and the write power P.sub.W, respectively, the time interval required for the readout power P.sub.R to charge to the write power P.sub.W when the optical recording and reproducing device is switched from the readout mode to the write or recording mode is dependent upon the response characteristics (frequency characteristics) of the power control circuits or the like. As a result, the rise time; that is, the time required for the readout power P.sub.R to rise to the write power P.sub.W, becomes longer. This slow rise time adversely affects an optical recording and reproducing device or system in which the power or intensity of laser light emitted from a light source or a semiconductor laser must be switched depending upon the readout or reproduction mode or the write or recording mode. More specifically, there arises a serious problem that because of a slow rise time; that is, the time interval required for the readout power P.sub.R to rise to the write power P.sub.W, it becomes impossible to record positively the leading portion of the information to be recorded. To put into another way, data or information at the beginning portion of a series of data or information to be recorded would be missed.